The present invention is directed to providing connections between polysilicon wiring and source and drain diffusions in CMOS technology.
In conventional NMOS technology, a buried contact is frequently used to provide a contact from polysilicon wiring to the source and drain diffusions. After the gate oxide has been formed, but before the polysilicon wiring is deposited, contact holes are etched in the gate oxide to selectively allow the polysilicon to be deposited directly on the silicon substrate. During the drive-in of the dopant into the polysilicon, the doping impurities in the polysilicon diffuse through the contact hole into the silicon substrate. Thus, when the conductivity type of the polysilicon wiring is the same as that of the source and drain diffusions, a self-aligned interconnect may easily be formed.
However, in a CMOS device which uses n-type polysilicon to form the gate of both p channel and n channel devices, this technique would not permit formation of a buried contact to a p+ source or drain diffusion. The n+ dopant diffusing out of the polysilicon would not form a connection to the p-type source or drain diffusion, but would form a short circuit to the n-type substrate or n-type well. This inability to form a buried contact to a p-type source or drain diffusion places a serious constraint on CMOS design, since the possibility of making direct contacts to both p-type and n-type source or drain areas would permit greatly increased efficiency of utilization of silicon real estate in CMOS design.
It is of course known, from Schottky TTL technology, to provide silicide strap connectors across p and n regions. However, in this technology Schottky barrier contacts, and not ohmic contacts, are formed to the n regions. It is also known to use platinum silicide to form ohmic source contacts, but it is believed to be novel to use silicides for ohmic contacts from poly to both p and n regions, according to the present invention.
Thus, it is an object of the present invention to provide a technology which permits direct formation of first contacts between a polysilicon gate-level interconnect and a source or drain region of either conductivity type.
It is a further object of the present invention to provide a technology which permits easy formation of ohmic contacts between polysilicon gate-level interconnects and source or drain regions of either conductivity type.
It is a further object of the present invention to provide a technology, for forming ohmic contacts to source or drain regions of either conductivity type, which is a self-aligned process.
It is a further object of the present invention to provide a technology, for forming connections between gate-level interconnects and source or drain regions of either conductivity type, which provides contacts which are as compact as those produced by a buried contact connection between an n+ doped polysilicon interconnect and an n-type source or drain region.